This invention describes a method and apparatus to scan and digitize images, such as photographic prints, transparencies or object produced images. Most digital image processing systems depend on the conversion of an image from its photometric density to digital values. The customary apparatus to measure these values from transparency or opaque images has been either a flying spot scanner, laser scanner or image scanning microdensitometer. These methods sweep a suitably chosen light spot over the image and detect the transmitted or reflected intensities. This detected value is a direct measure of the image density or gray level. Thus an analog-to-digital conversion of this electronic measurement provides the desired digitized value. A line-by-line scanning and detection in this manner allows digital image reproduction for applications such as image enhancement, change detection or video telecommunication.
A universal deficiency of image digitizers is the shape and intensity distribution of the scanning spot: in previous non-coherent scanning instruments, the scanning spot (or scanning aperture) has spatial frequency characteristics of degraded response below maximum frequency values of interest and non-zero response above these values. This in turn causes aliasing, excessive noise content and modulation of the desired sampled data. Previous instruments are restricted to the utilization of non-negative intensitity distributions, which is a fundamental limitation of many optical systems. With the exception of coherent scanning techniques, only limited success in removing these digitizing deficiencies has been achieved.
The elimination of aliasing and noise, as well as sharp limiting frequency cutoff can be achieved by a unique utilization of positive and negative scanning apertures as described in this invention. The shape and intensity distribution of these apertures have two generally desired configurations, however special applications will sometimes dictate other forms. This is specifically desirable for image processing in which considerable computation has previously been required to correct the sampled data. In many cases these distortions in the digitized data cannot be removed.
The application of incoherent optical techniques to achieve positive and negative components has been applied to the science of optical correlation. In this respect, relatively large area patterns are compared with the image to determine the presence of particular image patterns. The invention described herein is distinct from these area correlation instruments: area correlation instruments are specifically designed to detect the presence of image patterns irrespective of position or orientation, on a large or macro scale. This invention, conversely, provides optimized image digitizing using suitable chosen scanning spots or apertures on a microdensitometer scale. The correlation instrument measurement is a correlation value, whereas the digitizer instrument measurement is a series of density measurements.